The present invention relates to a method for manufacturing a glass substrate, and more particularly, to a method for manufacturing a glass substrate of an information recording medium, such as a magnetic disk, a magneto-optic disk, or an optical disk, a display apparatus, such as a liquid crystal or an organic EL display, a photomask, and various optical instruments.
In a prior art method for manufacturing a glass substrate, a glass plate having a predetermined shape is prepared. Then, the surface of the glass plate is polished to manufacture a glass substrate, which is flat and free from microscopic scratches. A polishing agent containing cerium oxide is normally used during the polishing. This is because cerium oxide exhibits strong chemical reactions with glass material, and the polishing amount per unit time is high. When using the polishing agent containing cerium oxide, the chemical reaction between the cerium oxide and the glass material may result in residual particles of cerium oxide on the surface of the glass plate. The residual cerium oxide particles on the surface of the glass plate lower the quality of the glass substrate. Therefore, the glass plate is washed after the polishing to remove the cerium oxide particles from the glass plate.
Normally, when washing the glass material, an alkaline aqueous solution, in which a surfactant and/or chelate agent are added, is used as a washing agent. However, in the alkaline aqueous solution, the chemical effect of cerium oxide, which is an inorganic substance, is weak. Thus, the cerium oxide particles cannot be sufficiently removed from the surface of the glass plate. Accordingly, when using the alkaline aqueous solution, the washing time must be extended or the washing temperature must be increased. However, the extension of the washing time and the adjustment of the washing temperature are not simple and decrease productivity.
In addition to the washing with the alkaline aqueous solution, an acidic aqueous solution, such as hydrofluoric acid or hydrosilicofluoric acid, may also be used to perform washing. In this case, the etching effect of the alkaline aqueous solution and acidic aqueous solution slightly dissolves the surface of the glass plate. This removes the residual cerium oxide particles. However, the etching effect roughens the surface of the glass plate and, as a result, decreases the quality of the glass plate. This is because the etching effect may expose small scratches (polishing marks) from the surface of the glass plate or may produce potential scratches.
A mixture of a reducing agent, which functions to decompose cerium oxide, and an acidic aqueous solution may also be used as the washing agent. This produces less potential scratches in comparison to when using only acidic aqueous solution as the washing agent. Further, this dissolves and removes cerium oxide from the surface of the glass plate. However, to remove the cerium oxide embedded in the glass plate, the glass plate must be washed for a long time. This decreases productivity.
The washing may also be performed through a plurality of processes, as described below. This method includes a first washing process using acidic aqueous solution, to which a reducing agent is added, and a second washing process, which uses only acidic aqueous solution, such as hydrofluoric acid or fluorosilicic acid. The cerium oxide on the surface of the glass plate is removed in the first washing process. The cerium oxide embedded in the glass plate is removed through etching in the second washing process. However, potential scratches may be produced in the second washing process.
Since the demand for a high quality glass substrate having a flat and smooth surface must be satisfied, there is a tendency of using an polishing agent with grains having a small diameter and applying a higher polishing pressure. High polishing pressure adheres the cerium oxide particles strongly to the glass plate and further deeply embeds the cerium oxide particles in the glass plate. In such a case, the cerium oxide particles cannot be removed sufficiently, even when using a reducing agent.